Method of working alloy steel and products thereof



Patented Sept. 11, 1945 UNITED METHOD or WORKING ALLOY s'rsm. AND

raonuc'rs 'rnsanor Henry S. Schaufus, Baltimore, Md., assignor to Rustless Iron and Steel Corporation, a corporation of Delaware No Drawing. Application December 13, 1940, Serial No. 370,073

Claims.

My invention relates to high chromium alloy steel products, especially of the character described in my co-pending application Serial No.

370,072 entitled Alloy steel and articles, filed of of good machining qualities and good impact strength, fashioned of high chromium alloy steel, particularly where fashioned of a high chromium, high sulphur containing steel of free-machining characteristics, and to the assuring in such products this combination of qualities in combination with good working properties and good corrosionresisting properties.

Other objects of my invention in part will be obvious and in part pointed out hereinafter.

The invention, therefore, consists in the several operational steps and the relation of each of the same to one or more of the others, as well as in the products achieved thereby, as more particularly described herein, the scope of the application of which is indicated in the claims.

In order to gain a fuller appreciation of certain features of my invention it should be noted at this point that at the presenttime there are a great many types of alloy steel in common use. For example, in the field of corrosion resisting steels there are perhaps some forty or fifty standard types available. All of these steels are referred to as stainless steels.

Within the field of stainless steels there are several types which are particularly suited for applications where machining operations are necessary in fashioning the product or article of ultimate use. These steels commonly include the iiigredient sulphur in substantial amounts. Itwill be understood, however, that no one type of these steels is suited for every class of service, the particular type employed being largely determined by the service conditions.

One of the types of stainless steel found useful in a number of general applications is the 12% to 14% chromium grade including, of course, sufficient sulphur to lend desired free-machining qualities. This is useful only where mildly corrosive conditions are encountered. The steel is readily workable through conventional hot or hot and cold methods into bars, rods, wire, plate, sheet, strip and the like. It may be heat-treated to give a. good range of hardness, strength and impact resistance. The steel, however, has a qualities.

rather serious shortcoming in that it is not suited for applications involving corrosion by mild acids, bases or salts, as in the handling of foods, fruits, mild chemicals or the like, or even where corrosive atmospheric conditions are encountered.

It reasonably might be expected that the desired corrosion resisting properties might be achieved in a related type of free-machining stainless steel through a substantial increase in the chromium content. I find, however, that such a steel, for example the 17% chromium grade, is seriously afiected by including sufilcient sulphur to lend it the necessary free-machining The steel, although corrosion resist= ing and of desired excellence in machlnability, is so poor in impact resistance that it is virtually useless for common engineering applications. It is in no sense suitable for various machine parts, products, articles, and devices where shock, vibration, impact or like stresses are encountered. It will be appreciated that this shortcoming is so serious as to make it necessary to go to other types of steel for such applications.

The principal type of free-machining stainless steel employed where common food and fruit acids ticularly by virtue of its high nickel content. The

steel has other shortcomings in that it is austenitic and accordingly substantially unhardenable by heat-treatment. It lacks the wear-resisting qualities which are-desirable in various machine parts, even though it is fully capable of withstanding the shock, vibration and other stresses encountered in use.

In my co-pending application noted above I provide an improved type of free-machining stainless steel-of good impact strength. I find, however, that in this steel certain variations in impact strength ordinarily occur when the converted products, that is, plate and sheet and bars, rods and wire, are worked through conven tional methods. Although the impact strength is substantially improved over known stainless steels of like chromium'and sulphur contents, still it leaves something to be desired.

Accordingly, one of the objects of the present invention is to provide a manner of economically and reliably assuring converted products of uniformly high impact strength without sacrifice of workability on the one hand or freemachining qualities on the other.

In the practice of my present invention the freemachining stainless steel containing 14.5% to 18% chromium, 20% to 50% sulphur, .5% to 2.0% nickel, with a maximum carbon content of 20%, preferably .-12% maximum. and the remainder principally iron and the usual impurities, is obtained, as more particularly indicated in my copending application referred to above. This steel in the form of ingots is cogged down in the usual manner. In accordance with my invention the resultant billets or blooms are converted into bars. rods and wire on the one hand or plate and sheet on the other, with precise temperature control and with a particular amount of reduction. Conventional hot-working equipment may be employed.

I find that best results in impact strength are achieved by conducting the hot-working operation at a comparatively low temperature. The billets or blooms to be converted should be started through the mill at a temperature no greater than about 2000 F. for bar stock, although in the case of wire where the percentage of hot reduction is extremely high, this temperature may .be exceeded somewhat. The products should be finished at a maximum temperature of about 12.00

F. and preferably at. a temperature no greater than 1100" F. The extent of the reduction should be at least 80%. The products then are reheated to a temperature of 1000 F. to 140 F. and aircooled. The structure is found to be decomposed martensite and ferrite, the ferrite amounting to some 60% to 80% of the whole.

I find my working temperatures are in every sense critical. Where the finishing temperature is appreciably exceeded there is an abrupt loss in impact strength of the product. Similarly, where in the composition of the metal without adversely ail'ecting the hot-working properties, the machinability or the impact strength. Thus, manganese, which like silicon is commonly found as an impurity in stainless steel, may purposely be added up to the. amount of 1.5%. The silicon content desirably is restricted to a maximum of about 35%, although where the chromium is on the low side and the nickel and manganese are on the high side the silicon may amount to as much as .60%. The manganese is beneficial in the hotworking operations, as well as in the steel melting process.

The products are further benefited by including in their composition the ingredient molybdenum in the amount of some .2% to 2.0%, best results generally being bad where the molybdenum content is less than about 1.0%. This ingredient lends a certain soundness to the structure of the metal.

Where desired, it will be understood that the ingredient nitrogen in the amount of .06% to .30%

the starting temperature is substantially exceeded, a like loss in impact strength also occurs.

As a specific example of the practice of my invention, the following comparison is given between two samples of free-machining stainless steel of good corrosion resisting properties analyzing 16.25% chromium, .31% sulphur, .'l6% nickel, .39% silicon, .63% manganese, .41% molybdenum, .07% carbon and the remainder iron. Both were rolled from a'three inch square billet to a one inch round bar. As a matter of convenience the comparative data is presented in tabular form.

- Rolling temperatures Structure Grain impact Start Finish F. F. FL-lbs. 2,210 1,700 60% ferrite remain- Sllghtlyelongated. 6-12 der decomposed martensite. 1,925 1,000 60% ferrite remain- Greatly elongated. 41-46 der decom martensite.

From the comparison given about it will be seen that although both samples are of identical analysis and both are reduced to the same extent,

. yet the one where controlled working temperatures are employed possesses an impact strength of at least three to four times that of the other. In other respects it is found that both roll satisfactorily, both are corrosion resisting and both machine well.

' While bars, rods. wire, plate, sheet and the like, according to my invention, may consist only of the ingredients chromium, sulphur, nickel, carbon and iron in the particular critical proportions set forth above, I find that other ingredients of certain particular amounts also may be included also may be included in the composition of the steel without adversely affecting the hot-working properties, all as more particularly described and claimed in my co-pending application, Serial No. 370,074, filedcf even date herewith and entitled Alloy steel method and products.

It will thus be seen that there has been provided in my invention a simple, direct and economical manner of achieving converted products of desired free-machining qualities in combination with high impact strength. Moreover, it will be seen that the products are strong, tough and corrosion resisting and are well-suited for fashioning into a wide variety of articles and products for engineering applications where shock, vibration, torsion, bending or like stresses are encountered in actual practical use.

As many possible embodiments may be made of my invention and as many changes'may be made in the embodiments hereinbefore set forth, it is to be understood that all matter described herein is to be interpreted as an illustration and not as a limitation.

I claim as my invention:

1. In an art of the class described, the method of producing bars, ,rods, wire, plate, sheet, and the like, of good machinability and high impact strength of alloy steel containing 14.5% to 18% chromium, 20% to .50% sulphur and the remainder substantially all iron, which consists in including in the composition of the steel the ingredient nickel in the amount of .5% to 2.0%;

working the products at a starting temperature of less than 2000 F., and finishing the working iron of good machinability and high impact strength, which consists of working the products at a starting temperature of less than 2000 F., and finishing the products at a temperature below 1200 F., the extent of the reduction being at least 80%.

3. man art of the class described, the method of imparting improved impact strength to bars, rods, wire, plate, sheet, and the like, of good free-machining qualities fashioned of an alloy steel containing 14.5% to 18.0% chromium, .20% to .50% sulphur, .5% to 2.0% nickel, up to 1.5% manganese, a maximume 01 35% silicon, a maximum of .20% carbon, and the remainder substantially all iron, which includes the step of working the products at a starting temperature below 2000 F. and finishing the products at a temperature below 1200 F.

4. In manufactures of the class described, high chromium alloy steel bars, rods, wire, plate, sheet, and the like of good machinability and high impact strength fashioned of a steel comprising 14.5% to 18.0% chromium, 20% to'.50% sulph r,

.5% to 2.0% nickel, a. maximum of .20% carbon,

with the remainder substantially all iron, worked 15 at a,temperature below 2000 F. and finished at a working temperature under 1200 F.

g 5. In manufactures of the class described, high chromium alloy steel bars, rods, wire, plate, sheet,

and the like of good machinability and high impact strength fashioned of a steelcomprising 14.5% to 18.0% chromium, 20% to .50% sulphur,

'.5% to 2.0% nickel, .2% to 2.0% molybdenum,

up to 1.5% .manganese, .35% silicon, a maximum of 20% carbon, with the remainder substantially all iron, worked at a temperature below 2000 F. and finished at a, working temperature under 1200 F., and air-cooled from a temperature of 1000 F. to 1450 F. s

, HENRY S. SCHAUFUS. 

